1. Field of the Invention
The present invention relates to a system and method for a MAC protocol that allows the collection of a large number of range measurements, and based upon such measurements, permits a very precise computation of the location of terminals inside a building. Specifically, the system and method operates with Mobile Terminals, Fixed References and at least one Main Control, wherein a MAC protocol defines the sequence of messages exchanged between such Mobile Terminals, Fixed References and the Main Control for assuring the data needed for computing the location is collected and correctly transferred.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the base node coverage area. The communications signals include, for example, voice data that has been sampled and modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc multi-hopping” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of expensive base stations. Details of an ad-hoc multi-hopping networks are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc multi-hopping networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with fixed nodes, such as those on the public switched telephone network (PSTN), and on other mobile or fixed networks such as cellular telephone networks and the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 B2 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 B2 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, granted on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 B2 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, granted on Mar. 29, 2005, the entire content of each application being incorporated herein by reference.
In either conventional wireless communications networks, or in ad-hoc wireless communications networks, it may be necessary or desirable for a mobile node to be capable of knowing or determining a relative or absolute geographic location or position. As known to those skilled in the art, this can be achieved through the use of a number of technologies. These technologies can use cell identification, combined with Round Trip Time (RTT), Timing Advance (TA) and Measured Signal level (RX level), Time Difference of Arrival (TDOA) and Angle Of Arrival (AOA) techniques, the details of which can be appreciated by one skilled in the art. Another available technology uses cellular signal timing based methods for code division multiple access (CDMA) and wideband code division multiple access (WCDMA). Yet another technology uses Global Positioning System (GPS) techniques, which is generally viewed as being more accurate than all other methods listed.
Despite the fact that the GPS technique has been in use for a considerable period of time and most of the world's navigation relies on this technique, the GPS technique is very susceptible to errors in measurement. Therefore, the GPS technique is capable of providing location determination results with very high accuracy only after performing a relatively large number of measurements to remove such errors. A description of the shortcomings of GPS is set forth in a document by the Institute For Mathematics and its Applications (IMA) entitled “Mathematical Challenges in Global Positioning Systems (GPS)”, the entire content of which being incorporated herein by reference. Other tests also demonstrate that the GPS technique is unsuitable for those terrestrial-based networks operating in locations where the number of simultaneous visible satellites is too small or not existent, like in underground tunnels, inside buildings, or in urban “canyons”.
To overcome the above issues with determining location information, ad-hoc networks are being developed which do not require either the use of satellites or a centralized computing facility for determining location information. Further details of such ad-hoc networks are described in U.S. Pat. No. 6,728,545 entitled “System and Method for Computing the Location of a Mobile Terminal in a Wireless Communications Network”, the entire contents of which is incorporated herein by reference. Additionally, ad-hoc multi-hopping networks can be developed utilizing non-fixed, or movable infrastructure components. Further details of networks using movable access points and repeaters for minimizing coverage and capacity constraints are described in U.S. Pat. No. 7,206,294 entitled “Movable Access Points and Repeaters for Minimizing Coverage and Capacity Constraints in a Wireless Communications Network and a Method for Using the Same”, granted on Apr. 17, 2007, the entire content being incorporated herein by reference. The precision of computed location with methods using Time Of Flight (TOF) as a measurement of the distance between terminals, is very dependent on the precision of the TOA. A method for improving the precision of the TOA is described in published U.S. Pat. No. 7,054,126 entitled “System and method for improving the accuracy of time of arrival measurements in a wireless ad-hoc communications network” granted on May 30, 2006 the entire contents of which is incorporated herein by reference.
The publications discussed above generally relate to mobile networks that connect to a permanent fixed network. However, as can be appreciated from the information referenced above, wireless ad-hoc multi-hopping networks do not necessarily have the same requirements, and include numerous communication issues that must be addressed in position determination. Accordingly, a need exists for a system and method for easily communicating the information required for calculating absolute and/or relative location of a mobile node.